In communication systems, information may be transmitted from one physical location to another. Furthermore, it is typically desirable that the transport of this information is reliable, is fast and consumes a minimal amount of resources. One of the most common information transfer mediums is the serial communications link, which may be based on a single wire circuit relative to ground or other common reference, multiple such circuits relative to ground or other common reference, or multiple circuits used in relation to each other. An example of the latter utilizes Differential signaling (DS). Differential signaling operates by sending a signal on one wire and the opposite of that signal on a paired wire; the signal information is represented by the difference between the wires rather than their absolute values relative to ground or other fixed reference.
Differential signaling enhances the recoverability of the original signal at the receiver, over single ended signaling (SES), by cancelling crosstalk and other common-mode noise, but a side benefit of the technique is that the Simultaneous Switching Noise (SSN) transients generated by the two signals together is nearly zero; if both outputs are presented with an identical load, the transmit demand on its power supply will be constant, regardless of the data being sent. Any induced currents produced by terminating the differential lines in the receiver will similarly cancel out, minimizing noise induction into the receiving system.
There are a number of signaling methods that maintain the desirable properties of DS while increasing pin efficiency over DS. Many of these attempts operate on more than two wires simultaneously, using binary signals on each wire, but mapping information in groups of bits. For example, a communication system may, for some k>1, map each k information bits of the set {0,1}k to a set C comprising 2 k code words. Each code word may have the same length and if that length is less than 2 k, the pin efficiency would be greater than 0.5. For example, each component may be conveyed on one of N wires and have coordinates belonging to a set {a, −a} so that each of the N wires carries a binary signal. For simple “repetitive” DS, the DS signals are applied independently to pairs of wires, so number of wires (N) would be 2 k. This mapping (with N<2 k) can provide higher pin efficiency relative to DS. Also, unlike “repetitive” SES, the set C does not contain all possible vectors of the given length.
Vector signaling is a method of signaling. With vector signaling, a plurality of signals on a plurality of wires is considered collectively although each of the plurality of signals may be independent. Each of the collective signals is referred to as a component and the number of plurality of wires is referred to as the “dimension” of the vector. In some embodiments, the signal on one wire is entirely dependent on the signal on another wire, as is the case with DS pairs, so in some cases the dimension of the vector may refer to the number of degrees of freedom of signals on the plurality of wires instead of exactly the number of wires in the plurality of wires.
With binary vector signaling, each component takes on a coordinate value (or “coordinate”, for short) that is one of two possible values. As an example, eight SES wires may be considered collectively, with each component/wire taking on one of two values each signal period. A “code word” of this binary vector signaling is one of the possible states of that collective set of components/wires. A “vector signaling code” or “vector signaling vector set” is the collection of valid possible code words for a given vector signaling encoding scheme. Stated mathematically, binary vector signaling maps the information bits of the set {0,1}k for some k>1 to a code, C, comprising 2 k vectors. Each vector may have the same dimension, N, and that dimension (i.e., number of components) may be greater than k but less than 2 k (resulting in a the pin efficiency above 0.5). A “binary vector signaling code” refers to a mapping and/or set of rules to map information bits to binary vectors.
With non-binary vector signaling, each component has a coordinate value that is a selection from a set of more than two possible values. A “non-binary vector signaling code” refers to a mapping and/or set of rules to map information bits to non-binary vectors. Stated mathematically, given an input word size, k>1, given a finite set, S, called the alphabet, of three or more values (possibly corresponding to, or representable by, values that may correspond to physical quantities as explained herein, wherein the values are typically real numbers), and given a vector dimensionality, N, non-binary vector signaling is a mapping between {0,1}k and a vector set, C, where C is a subset of SN. In specific instances, there is no proper subset T of S such that C is a subset of TN, i.e., over the code C, there is at least one component (coordinate position) in which each of the values of finite set S appears. In this case, S may be called the “true alphabet” of the code. For S being a finite set of three values, that means that there will be at least one coordinate position for which at least three code words all have different values. Any suitable subset of a vector signaling code denotes a “subcode” of that code. Such a subcode may be a vector signaling code.
Some vector signaling methods are described in Cronie I, Cronie II, Cronie III, and Cronie IV. For example:                (1) Orthogonal differential vector signaling is described, wherein the code C is obtained as the Hadamard transform images of vectors of length k with coordinates in the set {+1, 1};        (2) Permutation modulation codes are described, wherein the code C is obtained by taking the set of all vectors obtained from all permutations of a fixed vector; and        (3) Sparse signaling codes are described, wherein the code C is the set of all vectors obtained from all permutations of a fixed vector in which many coordinates are zero (or a fixed number).        
While non-binary vector signaling methods can provide substantial improvements regarding the tradeoff of pin efficiency, power efficiency and noise resilience as compared to traditional signaling methods, there are some applications wherein additional improvements are possible.